1. Field of Invention
This invention relates to communication systems, methods of operation and program products. More particularly, the invention relates to dynamic measurement of communication channel characteristics using direct sequence spread spectrum (DSSS) systems, methods and program products.
2. Description of Prior Art
A common problem in communication system is improving data transmission reliability between sending and receiving stations. Data transmissions are subject to bandwidth limitations, propagation impairments, noise and other factors. A communication channel which includes means to perform on-the-fly analysis to measure and adjust the channel characteristics for improved reliability would solve a long-standing problem in communication systems. One communication system which measures communication characteristics during transmission is a Direct Spread Spectrum System (DSSS) described in the text Wireless LANS: Implementing Interoperable networks by J. Geir, published by Macmillan Technical Publishing, 1999 at pages 47–49, 67–68 [(ISBN 98-85498)] (ISBN 1-57870-081-7). The measurement is taken for purposes of signal acquisition in synchronizing the Pseudo Noise (PN) codes at the sending and receiving stations. What is needed in the art is a DSSS system which enables the reliability of data transmission in a communication channel to be judged and the transmission adjusted accordingly for improved signal reliability.
Prior art related to DSSS communication systems includes:
(1) An article entitled “A New Serial Search Acquisition Approach With Automatic Decision Threshold Control” by S. Chung, published 1995 IEEE 45th Vehicular Technology Conference, V2, pages 530–536 discloses a CDMA signal acquisition system and uses, in each dwell, an adaptive signal threshold for signal detection and an adaptive signal classification threshold for classifying a signal versus noise. The system uses the detected signal and noise statistics to estimate SNR. The signal detection threshold is obtained from the maximum correlation value of the acquisition process. The signal classification threshold is derived from the average value of the signal detection threshold and noise estimate. If a new signal candidate is detected, both the signal detection threshold and the signal classification threshold are updated. As a result, the signal classification threshold is equivalent to the threshold used in the conventional serial acquisition system. The decision to stop or continue the acquisition search is aided by a pre-verification process.
(2) U.S. Pat. No. 6,052,408 issued Apr. 18, 2000 discloses improving the performance of a cellular communication system using direct sequence spread spectrum techniques. An apparatus and process enable dynamic modification of communication system parameters including PN code length, chipping rate and modulation technique for transmission of a data packet. Modification is based on proximity of the transmitter and receiver, noise level, transmitter and receiver capabilities and other factors. The system makes tradeoffs between data transmission speed and communication range to improve system performance.
(3) U.S. Pat. No. 5,732,111 issued Mar. 23, 1998 discloses a frequency error calibration, estimation and compensation device and method for direct sequence spread spectrum systems, such as digital cordless telephones having a handset and a base station. Initial frequency calibration occurs when the handset is physically coupled to the base station. The allowable frequency error range is divided into a plurality of frequency bins. Both the handset and base station transmit and receive messages using a predetermined frequency offset bin as an estimate of the frequency error. A signal quality value is determined and then a next bin is selected and a next signal quality value is determined. Once a signal quality value has been determined for each frequency bin, a communications link is established between the handset and the base station to allow automatic frequency control tracking to fine tune the frequency offset value. If the signal quality is not above a threshold value, the procedure is repeated. After system calibration, frequency error estimation is performed by cross product demodulation of the received in-phase and quadrature signals. The cross product output is limited and filtered. A phase rotator performs frequency compensation by multiplying the received in-phase and quadrature signals by (cos.phi.(t)+jsin.phi.(t)) where. phi.(t) is the sum of the current phase value and the limited and filtered cross product value.
None of the prior art discloses dynamic measurement of a communication channel using DSSS by extracting a PN code from a received signal using a correlator value and comparing the correlator value to a threshold where a correlator value below the threshold is indicative of unreliable communication in the channel and a correlator value above the threshold indicative of reliable transmission through the channel, the transmission characteristics of the signal being adjusted according to the correlator value—threshold comparison.